dbus_new_handlers: Omit errant dbus_message_unref

dbus_new_handlers: Omit errant dbus_message_unref

This unref is guaranteed to be freeing a NULL pointer.

Tested manually: use dbus-send to send an invalid debug level parameter

Before change:

$ dbus-send --system --dest=fi.w1.wpa_supplicant1 --print-reply
/fi/w1/wpa_supplicant1 org.freedesktop.DBus.Properties.Set
string:fi.w1.wpa_supplicant1 string:DebugLevel variant:string:msgdumpf
Error org.freedesktop.DBus.Error.NoReply: Message did not receive a reply
(timeout by message bus)
(and then wpa_supplicant crashes)

After change:

$ dbus-send --system --dest=fi.w1.wpa_supplicant1 --print-reply
/fi/w1/wpa_supplicant1 org.freedesktop.DBus.Properties.Set
string:fi.w1.wpa_supplicant1 string:DebugLevel variant:string:msgdumpf
Error fi.w1.wpa_supplicant1.InvalidArgs: Did not receive correct message
arguments.

Signed-off-by: Paul Stewart <pstew@google.com>